You know the definition if you looked it up but if you want to know how they are related, I believe that would be the answer but I do not practice it so I cannot give you a completely informed relation. =-)

having a man watch as his woman is screwed by another man, often roughly and sometimes with the woman moaning how much better that man is than her husband, is cuckold. So what that person was telling you was that you are becoming your girlfriend's bitch. It wasn't a compliment.